FIRST:
I started by working out dimensions for my box, cutting it all and then I put it together except for the top and back. Then I cut out the wholes for the head unit and speakers and I mounted them.

After that I wired the speakers to the head unit loom.
Then I started preparing the power supply.

SECOND:
for me this was the hardest part to understand of the other instructions, so I will do my best to describe.
Its pretty simple, get all the wires from the power supply and undo them. Then sort them by colour.
Strip the ends of the yellow (+12V) and the black (ground) wires.
The yellow wires need to go to the red and yellow wires on the head unit loom.
The black wires need to go to the black wire on the head unit loom.
Keep one black wire separate and attach it and the green wire to a switch.
If the green wire is not wired to a black wire, the power wont turn on.

Cover the ends of all the other wires and manage them as you wont be needing them
turn it on, see if it works!

THIRD:

if its all working,
next I glued my power supply down, using tarzans' grip (super glue).
After that was dry I cut out a hole in the back to allow air flow from the power supply fan.
Then I mounted the antenna inturnaly. I attached the switch, this was hard but I got there in the end.
Then I attached the handles (this thing is heavy).

After all that, you are ready to screw the back and top down.

And you now have a LOUD stereo.

If I have left anything out, or you have any questions, please leave a comment!
I have not authored many instructables yet, so I appreciate any feedback.

I might end-up doing this with the radio I got for a car I'll be junking soon. (bad gas leak, engine is tired, frame rotted.).. Someone else posted a similar 'redneck boom-box' idea, I imagine using a Gel-Cell battery as the power, but I guess you could also do a DP3T switch, center off, one direction off the battery, other direction, turning on the ATX supply, and applying voltage to the battery to trickle-charge it. (unless you fiddle with the overvoltage & regulators in the supply to push the +12 to no higher than +14)